The invention relates to a method for manufacturing very thin strips with a thickness less than or equal to 12 xcexcm, in an alloy of the aluminum-iron type. Such strips are notably used for manufacturing multilayer composites including a paper or cardboard layer, an aluminum alloy layer and a polymer layer, which may be used for making aseptic flexible or stiff food packages of the brick type.
The sought-after properties of use for very thin strips of aluminum alloy are good mechanical strength, sufficient elongation, a very low number of holes per unit surface and good tear and bending strength. The absence of holes is essentially related to the grain size which should in any case be less than the final thickness.
Moreover, as regards the industrial manufacturing of the product, it is important that the selected alloy may be cast and rolled easily, that it is not too costly to elaborate, notably that it does not require a too low silicon content and finally that the product""s processing range is not too complicated, in particular it should avoid a too large number of heat treatments.
The commonly used alloys for this application are alloys of the 1100 or 1200 type containing less than 1% by weight for the sum of silicon and iron contents. The use of higher iron alloys such as alloys 8006 and 8015, registered at the Aluminum Association in 1978 and 1988, respectively, and the addition of manganese are also known to improve mechanical strength.
The registered composition for 8006 is (% by weight):
Si less than 0.4 Fe:1.2-2 Cu less than 0.30 Mn:0.3-1 Mg less than 0.10 Zn less than 0.10
The registered composition for 8015 is:
Si less than 0.30 Fe:0.8.2-1.4 Cu less than 0.10 Mn:0.10-0.40 Mg less than 0.10 Zn less than 0.10
An important drawback of high iron alloys is the difficulty in recycling the manufacturing scrap for other applications; indeed, the manufacture of very thin strips is a delicate operation which leads to an important yield while generating a lot of waste material. A means for avoiding this drawback is to use, for producing the strips, a continuous casting machine, for example, continuous casting between rolls, which enables the direct recycling of scraps and wastes from the process into the machine""s melting furnace. This advantage is added to the intrinsic advantages of continuous casting, notably the low investment costs.
U.S. Pat. No. 5,380,379, filed in 1993 on behalf of Alcoa Aluminio Do Nordeste, describes an aluminum strip with composition (% by weight):
Si less than 0.2 Fe:1.35-1.6 Cu less than 0.1-0.4 Mn:0.3-0.6 B:0.01-0.02 made by continuous casting between rolls, to a thickness between 4.8 and 10 mm, annealed at more than 450xc2x0 C. and cold-rolled. If the final thickness of the strip is less than 9 xcexcm, the patent recommends extra-intermediate annealing.
Patent EP 0750685 (Alcan International), filed in 1994, relates to a thin foil with a thickness between 5 and 40 xcexcm, with composition (% by weight):
Si less than 0.4 Fe:1.2-2.0 Mn:0.2-1.0 Mg and/or Cu:0.1-0.5 Zn less than 0.1 with an average grain size less than 5 xcexcm after final annealing. The metal may be cast by conventional semi-continuous casting or by continuous casting between rolls or between belts.
The WO 98/45492 Patent application (Alcan International) describes a recyclable thin foil, notably for household applications, with composition:
Si:0.2-0.5 Fe:0.4-0.8 Cu less than 0.1-0.3 Mn:0.05-0.3 containing at least 2% by weight of dispersoids and at least 0.1% of copper and/or manganese in a solid solution. The alloy is continuously cast and an intermediate annealing is performed during the cold-rolling.
The object of the invention is to provide a method for manufacturing aluminum-iron type alloy strips with a thickness less than or equal to 12 xcexcm, and preferably less than 9 xcexcm, by using continuous casting between rolls, and leading to strips having both good mechanical strength and high tear and bending strength, in technical and economical conditions compatible with large-scale industrial production. The object of the invention is a method for manufacturing aluminum alloy strips with a thickness less than or equal to 12 xcexcm, and preferably less than 9 xcexcm, including:
elaboration of an alloy with composition (% by weight):
Si:0.15-0.40 Fe:1.10-1.70 Mg less than 0.02 Mn:0.30-0.50
other elements less than 0.05 each and total less than 0.15, remainder is aluminum,
continuous casting between rolls of a strip of this alloy with a thickness between 2 and 10 mm,
homogenization of this strip at a temperature between 450 and 620xc2x0 C. with a duration between 8 and 40 hrs,
cold-rolling of this strip
intermediate annealing of the cold-rolled strip at a temperature between 200 and 400xc2x0 C., and with a duration between 8 and 15 hrs,
cold-rolling of the annealed strip to a final thickness less than or equal to 12 xcexcm,
final annealing of the strip at a temperature between 200 and 300xc2x0 C., with a duration of at least 50 hrs.